Image sensor arrays typically comprise a linear array of photosensors which raster scan an image-bearing surface or document and convert the microscopic image areas viewed by each photosensor to image signal charges. Following an integration period, the image signal charges are amplified and transferred as an analog video signal to a common output line or bus through successively actuated multiplexing transistors.
For high-performance image sensor arrays, one possible design includes an array of photosensors of a width comparable to the width of a page being scanned, to permit one-to-one imaging generally without the use of reductive optics. In order to provide such a “full-width” array, however, relatively large silicon structures must be used to define the large number of photosensors. One technique to create such a large array is to make the array out of several butted silicon chips. In one design, an array includes 20 silicon chips, butted end-to-end, with each chip having active photosensors spaced at 400 or more photosensors per inch.
In most scanning systems currently in use, the signal coming out of the photosensors during the scanning process is an analog video signal, with the voltage magnitude corresponding to the intensity of light impinging on the photosensor at a given time. The different video levels correspond to the brightness of the reflected area being scanned by a particular photosensor at a particular moment. These analog outputs are digitized and then applied, as needed, to look-up tables, such as to convert the signals to a device-independent color space for further processing.
In a scanner for recording color images, there is typically provided multiple linear arrays or rows of photosensors. Each array/row includes a translucent filter, causing the particular linear array/row to be sensitive to substantially one primary color, such as red, blue, and green, to correspond to structures in the human eye. The signals from each filtered linear array/row are then recorded for assembling a full-color image. There may also be a “white,” or effectively non-filtered, array/row. With a color input scanner, there is a need for occasional calibration of the linear array/row outputs relative to each other; to do this, the arrays/rows are typically exposed to white light from a test target of known properties, and their outputs are compared and generally normalized, such as through a look-up table for signal outputs. In a calibration operation, the look-up tables are typically altered depending on current conditions, to result in a normalized output when images are recorded.
U.S. Pat. No. 6,266,438 describes a basic color calibration system for a multi-chip input scanner, U.S. Pat. No. 5,373,374 describes an input scanner in which each of a set of primary-color filters are selectably placed in front of a single-linear-array photosensitive device, while U.S. Pat. No. 7,271,380 describes a calibration system for a photosensitive apparatus, such as a scanner, having a plurality of parallel linear-array photosensitive devices where each linear-array includes a primary color filter.
Although the above described calibration techniques are used with linear array systems, certain applications, such as high quality color printing on single marking engines or high quality color matching on dual marking engines, for example, Tightly Integrated Serial Printing (TSIP) or Tightly Integrated Parallel Printing (TIPP), require very precise absolute color detection and control, and preferably across the whole marking width and length. Single spot spectrophotometers have been used for inline or offline color fidelity control, usually in one process dimension or both directions with considerable effort, or alternatively with slow offline methods. For ease of use and integration, it is often desired to have a spectrophotometer inline with the print path, so that sheets, media or other image bearing surfaces are scanned automatically in real-time, with little or no user interaction. Full process sensors have been used to control relative color differences with some degree of accuracy, but these techniques are not able to control absolute color fidelity. Existing inline spectrophotometers (ILS) are relatively expensive and require extensive calibration techniques.
As can be derived from the variety of devices and methods directed at color printing quality control, many means have been contemplated to accomplish the desired end, i.e., consistent and reproducible color presentation within a single printing device as well as between a plurality of printing devices. Heretofore, tradeoffs between expense, accuracy, consistency and speed of measurement were required. Thus, the inventors have recognized that it would be desirable to provide an inline spectrophotometer which is cost effective to incorporate within a printing device that is capable of both controlling print quality within a single print device and between a plurality of print devices.